CN115537647B - High-toughness, low-yield ratio and low-longitudinal-transverse-strength anisotropic 600 MPa-grade steel plate and manufacturing method thereof - Google Patents

High-toughness, low-yield ratio and low-longitudinal-transverse-strength anisotropic 600 MPa-grade steel plate and manufacturing method thereof Download PDF

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CN115537647B
CN115537647B CN202110734971.9A CN202110734971A CN115537647B CN 115537647 B CN115537647 B CN 115537647B CN 202110734971 A CN202110734971 A CN 202110734971A CN 115537647 B CN115537647 B CN 115537647B
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刘自成
顾晔
张曦
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Baoshan Iron and Steel Co Ltd
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Abstract

A high-toughness, low-yield-ratio and low-longitudinal-transverse-strength anisotropic 600 MPa-grade steel plate and a manufacturing method thereof adopt a low-C-low-Si-medium Mn series low-alloy steel design, improve the Als content in the steel, ensure that Als/N is more than or equal to, low-carbon equivalent, ultralow-N content, low (Cu+Ni+Mo) alloying, nb microalloying-ultramicroTi treatment and Ca treatment are adopted, and optimize TMCP technology. The yield strength of the steel plate is more than or equal to 460MPa, the tensile strength is more than or equal to 570MPa, and the impact energy KV is more than or equal to minus 60 DEG C 2 (single value) is more than or equal to 120J, the yield ratio is less than or equal to 0.85, the longitudinal and transverse strength anisotropy is low (namely, the longitudinal/transverse strength difference is less than or equal to 30 MPa), and the weldability is excellent (preheating is not needed before welding, and the welding heat input is more than or equal to 50 kJ/cm); the method is particularly suitable for icebreaker shells, ocean platforms, cross-sea bridges, ocean wind power pile legs, wind tower structures, port machinery and the like in ice sea areas, and can realize low-cost stable batch industrial production.

Description

High-toughness, low-yield ratio and low-longitudinal-transverse-strength anisotropic 600 MPa-grade steel plate and manufacturing method thereof
Technical Field
The invention relates to low-carbon low-alloy steel, in particular to a 600 MPa-grade steel plate with high toughness, low yield ratio and low longitudinal and transverse strength anisotropy and a manufacturing method thereof.
Background
As is well known, low carbon (high strength) low alloy steel is one of the most important engineering structural materials, and is widely applied to petroleum and natural gas pipelines, ocean platforms, shipbuilding, hydroelectric engineering, bridge structures, boiler vessels, building structures, automobile industry, railway transportation and mechanical manufacturing; the properties of low carbon (high strength) low alloy steels depend on their chemical composition and manufacturing process, with strength, toughness, plasticity, weldability and the matching between them being the most important properties of low carbon (high strength) low alloy steels, which ultimately depend on the microstructure and dislocation substructure of the finished steel.
With the continuous forward development of metallurgical technology, people put forth higher requirements on low anisotropy of toughness, plasticity, particularly weldability and performance of high-strength steel, namely, the steel plate has brittle fracture resistance and plastic instability fracture resistance at a low temperature (-40 ℃), and meanwhile, the fracture elongation and uniform elongation reach the level of 500MPa grade steel plate, and the steel plate has excellent weldability and low anisotropy of performance and can bear high heat input welding (welding heat input is more than or equal to 50 kJ/cm); and the comprehensive mechanical property and the service performance of the steel plate are greatly improved under the conditions of lower alloy content, particularly precious metal content and lower manufacturing cost, so that the alloy consumption of the steel is reduced, the cost is saved, the self weight, the stability and the safety of the steel member are reduced due to high strength and light weight, and more importantly, the cold/hot workability and the safety and the reliability in the service process of the steel member are further improved.
At present, research on developing new generation of high-performance steel materials is carried out in the range of Japanese and Korean, european Union and North American, and better microstructure matching is obtained by alloy combination design optimization, control of a sub-microstructure fine structure and innovation of a manufacturing process technology, and superfine metallographic microstructure and a sub-structure (dislocation configuration and packet) fine structure are realized, so that high-strength steel is better in plasticity and plasticity matching, weldability and low anisotropy.
In the prior art, when a thick steel plate with the yield strength of more than or equal to 420MPa and the low-temperature impact toughness of more than or equal to 34J at minus 60 ℃ is manufactured, a certain amount of Ni or Cu+Ni element (more than or equal to 0.30%) is generally added into the steel. See [ The Firth (1986) international Symposium and Exhibit on Offshore Mechanics and Arctic Engineering,1986, tokyo, japan,354; "DEVELOPMENTS IN MATERIALS FOR ARCTIC OFFSHORE STRUCTURES"; "Structural Steel Plates for Arctic Use Produced by Multipurpose Accelerated Cooling System" (Japanese), kawasaki iron works, 1985, no. 1-72; "Application of Accelerated Cooling For Producing 360MPa Yield Strength Steel plates of up to 150mm in Thickness with Low Carbon Equivalent", accelerated Cooling Rolled Steel,1986, 209-219; "High Strength Steel Plates For Ice-Breaking Vessels Produced by Thermo-Mechanical Control Process", accelerated Cooling Rolled Steel,1986, 249-260; "420MPa Yield Strength Steel Plate with Superior Fracture Toughness for Arctic Offshore Structures", kawasaki steel technical report,1999, no.40, 56; "420MPa and 500MPa Yield Strength Steel Plate with High HAZ toughness Produced by TMCP for Offshore Structure", kawasaki steel technical report,1993, no.29, 54; "Toughness Improvement in Bainite Structure by Thermo-Mechanical Control Process" (Japanese) Sumitomo metal, vol.50, no.1 (1998), 26; "steel sheet for ocean platform construction used in The ice sea area" (Japanese), steel works, 1984, no. 314, 19-43) to ensure excellent low temperature toughness of The base steel sheet, and also enables toughness of The affected zone HAZ to reach AJ-60 ℃ of greater than or equal to 34 KJ/cm when The base steel sheet is welded with a heat input of < 50KJ/cm.
However, when welding with an ultra-large heat input (. Gtoreq.100 KJ/cm), the low-temperature toughness of the weld Heat Affected Zone (HAZ) is generally difficult to achieve, and the low-temperature toughness of the Heat Affected Zone (HAZ) is severely deteriorated. A large number of patent documents only describe how to realize the low-temperature toughness of a base steel sheet, and how to obtain excellent Heat Affected Zone (HAZ) low-temperature toughness under welding conditions is less, especially how to ensure low-temperature toughness of the Heat Affected Zone (HAZ) is less when ultra-large heat input welding is adopted, and in order to ensure the low-temperature toughness of the steel sheet, a certain amount of Ni or cu+ni element is generally added into the steel, and the low-temperature toughness of the Heat Affected Zone (HAZ) of the steel sheet is also less capable of reaching-60 ℃. (see Japanese Patent Sho 63-93845, sho 63-79921, sho 60-258410, japanese Patent Hei kai 4-285119, japanese Patent Hei 4-308035, hei 3-264614, hei 2-250917, hei 4-143246, US Patent4855106, US Patent5183198, US Patent 4137104).
Only Japanese new Japanese iron company, which has improved the low temperature toughness of the Heat Affected Zone (HAZ) of ultra-large heat input welded steel plates, adopts oxide metallurgy technology (see U.S. Pat. No. 4629505, WO 01/59167A 1), i.e. during the process of large heat input welding, tiN particles dissolve and lose effect due to long-time high temperature action in the vicinity of the weld line, ti 2 O 3 Is more stable than TiN and does not dissolve even if the melting point of the steel is reached. Ti (Ti) 2 O 3 The particles can become acicular ferrite core positions in austenite crystals, promote Acicular Ferrite (AF) nucleation in austenite crystals, effectively divide austenite grains, refine HAZ structures and form high-strength high-toughness acicular ferrite structures.
The Chinese patents ZL201410300713.X, ZL201310244712.3, ZL201310244706.8, ZL201310124065.2, ZL201310244713.8, ZL201210209637.2, ZL201410815614.5, 201710183350.X and 201910149978.7 disclose a series of low-temperature steel plates capable of being welded with large heat input, in order to ensure the low-temperature toughness of a heat affected zone of the large heat input welding, a certain amount of noble alloy elements Cu and Ni are added into the steel plates (particularly super-thick steel plates), when the yield strength YP reaches 460MPa, a small amount of Mo is particularly required to be properly added, the high-heat input welding manufacturability performance of the steel plates is better, but the toughness of the welding heat affected zone, particularly the low-temperature toughness of the welding heat affected zone of the thick steel plates is unstable (under the condition of large heat input welding, mo promotes the formation of coarse upper bainite, and deteriorates the toughness of the welding coarse crystal heat zone), the requirement of the impact toughness of the thick steel plates cannot be stably reached at-40 ℃, and the manufacturing cost of the thick steel plates is higher; more importantly, the developed TMCP steel plates do not relate to the anisotropy of the longitudinal/transverse strength and the control of low yield ratio of the steel plates, and the difference of the longitudinal/transverse strength of the steel plates reaches 50-120 MPa, so that the design safety requirements of special large-scale heavy steel structures (such as a large-span male iron bridge structure, an ocean engineering structure, a giant marine floating crane and the like) cannot be met.
Disclosure of Invention
The invention aims to provide a 600 MPa-grade steel plate with high toughness, low yield ratio and low longitudinal and transverse strength anisotropy and a manufacturing method thereofThe manufacturing method comprises the steps of impact energy KV with yield strength of the steel plate being more than or equal to 460MPa, tensile strength being more than or equal to 570MPa and minus 60 DEG C 2 (single value) is more than or equal to 120J, the yield ratio is less than or equal to 0.85, the longitudinal and transverse strength anisotropy is low (namely, the longitudinal/transverse strength difference is less than or equal to 30 MPa), and the weldability is excellent (preheating is not needed before welding, and the welding heat input is more than or equal to 50 kJ/cm); the low-temperature toughness of the HAZ during high heat input welding is also excellent, namely the impact toughness KV at the temperature of minus 40 ℃ of the base metal steel plate, while the high strength, excellent low-temperature toughness, low yield ratio and low longitudinal/transverse strength anisotropy of the base metal steel plate are obtained 2 Not less than 100J, welding Heat Affected Zone (HAZ) -40 ℃ KV 2 The method is more than or equal to 100J, is particularly suitable for icebreaker shells, ocean platforms, cross-sea bridges, ocean wind power pile legs, wind tower structures, port machinery and the like in ice sea areas, and can realize low-cost stable batch industrial production.
In order to achieve the above purpose, the technical scheme of the invention is as follows:
high toughness, low yield ratio and low anisotropy TMCP steel sheet is one of the most difficult varieties in thick plate products, because the steel sheet requires not only low C, low carbon equivalent CEV, high strength, excellent low temperature toughness, fatigue resistance, low yield ratio and low anisotropy, but also can withstand high heat input welding, and the weld heat affected zone has excellent low temperature impact toughness, but these performance requirements are hardly satisfied at the same time:
a) Low C, low carbon equivalent CEV with high strength, low yield ratio;
b) High strength, high toughness and low yield ratio, low anisotropy of longitudinal/transverse strength;
c) High strength and excellent weldability;
the above properties are conflicting with each other in composition design and TMCP process design and are difficult to reconcile.
When the C content and the carbon equivalent CEV are reduced, it is difficult to achieve high strength and low yield ratio of the steel sheet; the strength and the low-temperature toughness are improved, and meanwhile, the low yield ratio and the low longitudinal and transverse strength anisotropy of the steel plate are difficult to realize; when the steel sheet attains high strength, the weldability of the steel sheet, particularly, the large heat input steel sheet is hardly ensured. How to balance high strength, high toughness, low yield ratio, low anisotropy of longitudinal and transverse strength and high heat input weldability is one of the biggest difficulties of the product and is also a key core technology.
Therefore, the invention combines key factors of high strength, high toughness, low yield ratio, low longitudinal and transverse strength anisotropy, high heat input weldability and the like of the steel plate, and starts from alloy design, adopts low C-low Si-medium Mn series low alloy steel as a base, improves the content of Als in the steel as much as possible, and has Als/N more than or equal to 12, low carbon equivalent, ultra-low N content, low (Cu+Ni+Mo) alloying, nb microalloying-ultra-micro Ti treatment, ca/S ratio control between 1.0 and 3.0 and Ca multiplied by S 0.28 ≤1.0×10 －3 And the alloy is optimally combined and designed, and the TMCP process is optimized, so that the microstructure of the finished steel plate is uniform and fine ultra-low carbon bainite and a very small amount of massive ferrite, and the average grain size of the microstructure is below 20 mu m.
Specifically, the high-toughness, low-yield-ratio and low-longitudinal-transverse-strength anisotropic 600 MPa-level steel plate comprises the following components in percentage by weight:
C：0.060～0.10％
Si：≤0.20％
Mn：1.10～1.60％
P：≤0.013％
S：≤0.003％
Cu：0.05～0.25％
Ni：0.05～0.30％
Mo：0.08～0.25％
Nb：0.008～0.028％
Ti：0.008～0.016％
Als：0.040～0.070％
N：≤0.0050％
ca:0.0010 to 0.0035 percent; the balance of Fe and other unavoidable impurities; and the contents of the elements must satisfy the following relationships at the same time:
Als/N is more than or equal to 12; eliminating free N content in the welding heat affected zone, improving microstructure of the welding heat affected zone (forming high-density fine AlN particles, pinning austenite grains grow, refining microstructure of HAZ) and low-temperature toughness and crack-stopping property (eliminating free N of HAZ, eliminating free N embrittlement);
[ξ×(900-T start rolling )×(％Nb)]The ratio of H multiplied by zeta is less than or equal to 2.25, so that the steel plate has low yield ratio and low longitudinal and transverse strength anisotropy while high toughness is obtained; wherein H is the thickness of the finished steel plate and is in mm; xi is the cumulative rolling reduction of unrecrystallized controlled rolling in units; t (T) Start rolling The initial rolling temperature of unrecrystallized controlled rolling is the unit DEG C; ζ is the expansion ratio, namely the width of the finished steel plate/the width of the plate blank; this is one of the key core technologies of the present invention;
6≤[H×(T cooling -T Stop cooling )×CEV×ξ]/[(V c )×(T Stop cooling )]Not more than 665; the steel plate has high strength, high toughness and low yield ratio under the condition of low C and low carbon equivalent CEV, and meanwhile, the steel plate has excellent high heat input weldability, and the contradiction between the low C and low carbon equivalent CEV of the steel plate and the anisotropy of high strength, high toughness, low yield ratio and low longitudinal and transverse strength, which is difficult to be compatible, is successfully eliminated; wherein,,
h is the thickness of the finished steel plate, and the unit is mm;
T cooling The cooling temperature is set to be the unit of the initial cooling temperature for accelerated cooling;
T stop cooling Stopping cooling temperature for accelerated cooling at unit ℃;
CEV is steel sheet carbon equivalent, cev=c+mn/6+ (cu+ni)/15+ (cr+mo+v)/5 in units;
xi is the cumulative rolling reduction of unrecrystallized controlled rolling in units;
V c the unit is that the cooling speed is accelerated for the steel plate; this is one of the key core technologies of the present invention;
ca treatment, and Ca/S ratio is controlled to be 1.0-3.0 and Ca X S 0.28 ≤1.5×10 －3 : while ensuring vulcanization spheroidization and minimizing the influence of inclusions on low-temperature toughness and weldability, ca (O, S) particles are uniformly and finely distributed in steel, so that austenite grains in a large heat input welding heat affected zone are restrained from growing, the anisotropy of the longitudinal and transverse properties (strength and toughness) of the steel plate is reduced, and the low-temperature toughness of the large heat input welding heat affected zone of the steel plate is improved.
The component data in the above relation are calculated according to percentage, for example, the carbon content is 0.10%, and when the relation is calculated, the component data is carried into calculation by 0.10.
In the composition design of the steel plate of the invention:
c has great influence on the strength, low-temperature toughness, elongation and weldability of TMCP steel plates, especially the high heat input weldability, and from the viewpoint of improving the low-temperature toughness and the high heat input weldability of the steel plates, the C content in the steel is expected to be controlled relatively low; however, from the aspects of strength, low-temperature toughness, low yield ratio, low longitudinal and transverse strength anisotropy control, microstructure control and manufacturing cost in the production and manufacturing process of steel sheet, the content of C is not controlled to be too low; too low C content easily causes high yield ratio, and the mobility of the grain boundary is too high, so that the base metal steel plate and the welded HAZ microstructure are coarse in grains and easy to generate mixed crystals, and the C content in the steel is too low, so that the grain boundary is weakened, and the low-temperature toughness of the base metal steel plate and the welded HAZ is seriously deteriorated; therefore, the reasonable range of the C content is 0.06-0.10%.
Sipromotesdeoxidizationofmoltensteelandcanimprovestrengthofasteelsheet,butwithAldeoxidizedmoltensteel,Sihasasmalldeoxidizationeffect,andSicanimprovestrengthofthesteelsheet,butSiseriouslyimpairslow-temperaturetoughness,elongationandweldabilityofthesteelsheet,andparticularly,inthecaseofhigh-strengthsteelwithahighalloycontent,whenhighheatinputweldingisperformed,SipromotesformationofM-Aislands,theformedM-Aislandsarerelativelycoarseinsizeandunevenindistribution,andseriouslyimpairslow-temperaturetoughnessofaweldingheataffectedzone(HAZ),sothatSicontentinthesteelshouldbecontrolledtobeaslowaspossible,andSicontentiscontrolledtobe0.20%orlessinconsiderationofeconomyandoperabilityofasteelmakingprocess.
Mn as the most important alloying element in steel, in addition to improving the strength of the steel sheet, has the functions of enlarging the austenite phase region and reducing Ar 3 The spot temperature, the effect of refining TMCP steel plate crystal grains to improve the steel plate strength (fine grain strengthening effect), the effect of improving the low-temperature toughness (fine grain toughening) and anti-fatigue property of the steel plate, and the effect of promoting the formation of low-temperature phase transformation structure (phase transformation strength effect) to improve the steel plate strength; however, mn is liable to segregate during solidification of molten steel, and particularly when the Mn content is high, not only the casting operation is difficult, but also the Mn is liable to react with C,P, S, especially when the C content in steel is high, aggravates segregation and looseness of the central part of a casting blank, serious segregation of the central part of the casting blank easily forms abnormal structures in the subsequent TMCP and welding processes, so that the low-temperature toughness of a steel plate is low and cracks appear in a welded joint; therefore, the Mn content is suitably 1.10% to 1.60%.
P has a great damaging effect on the mechanical properties of steel, especially low-temperature impact toughness, elongation and weldability, as a harmful inclusion in the steel, and theoretically, the lower the requirement is, the better; however, considering steel-making workability and steel-making cost, the P content needs to be controlled to 0.013% or less for TMCP steel sheets which are required to be weldable with large heat input, toughness at-40 ℃, and high strength.
S is used as a harmful inclusion in steel, has great damage to low-temperature toughness, weldability and fatigue resistance of the steel, more importantly, S is combined with Mn in the steel to form MnS inclusion, and in the hot rolling process, the plasticity of MnS enables MnS to extend along the rolling direction to form a strip along the rolling direction of the MnS inclusion, so that the low-temperature impact toughness, the elongation, the Z-direction performance, the fatigue resistance and the weldability (particularly the high heat input weldability) of the steel plate are seriously damaged, and serious anisotropy of longitudinal and transverse strength and toughness is caused; meanwhile, S is a main element generating hot brittleness in the hot rolling process, and theoretically, the lower the requirement is, the better; however, considering the steel-making operability, steel-making cost and the principle of smooth material flow, for TMCP steel plates which are required to be welded with large heat input, the S content needs to be controlled to be less than or equal to 0.003 percent, and the TMCP steel plates are required to be welded with large heat input, and have high toughness at-40 ℃.
Cu is also an austenite stabilizing element, and Ar can be reduced by adding Cu 1 、Ar 3 The spot temperature is used for improving the atmospheric corrosion resistance of the steel plate, refining the microstructure of the TMCP steel plate and improving the low-temperature toughness of the TMCP steel plate; however, the Cu addition amount is too much and is higher than 0.45%, which not only easily causes problems of copper embrittlement, surface cracking and internal cracking of casting blanks, but also is particularly thickThe steel plate has the characteristics of impact load fracture resistance (namely plasticity and toughness) and the performance of a welded joint, and the steel plate has high yield ratio and high longitudinal and transverse strength anisotropy; while considering Cu as a relatively noble alloying element, the upper limit of Cu is preferably controlled to 0.30% in view of cost effectiveness. The addition amount of Cu is too small and is lower than 0.05 percent, and the Cu does not play any role basically; therefore, the Cu content is controlled between 0.05% and 0.25%.
The addition of Ni can not only reduce dislocation lattice friction force (namely P-N force) of a BCC crystal structure, improve low-temperature dislocation mobility of ferrite phase, promote dislocation slip and improve intrinsic ductility and toughness of ferrite; in addition, ni is used as a strong austenite stabilizing element, and Ar is greatly reduced 1 、Ar 3 The point temperature is increased, the driving force of austenite to ferrite transformation is increased, the austenite is transformed at a lower temperature, the microstructure of the TMCP steel plate is greatly thinned, the expansion resistance of cracks penetrating through ferrite grains is increased, and the low-temperature toughness of the TMCP steel plate is greatly increased, so that Ni has the effects of simultaneously improving the strength and the low-temperature toughness of the TMCP steel plate without reducing the elongation (namely, the plastic toughness); ni is added into the steel, so that the copper embrittlement phenomenon of the copper-containing steel can be reduced, the inter-crystal cracking in the hot rolling process is lightened, and the atmospheric corrosion resistance of the steel plate is improved. Therefore, in theory, the higher the Ni content in the steel is, the better, but the too high Ni content not only can harden the welding heat affected zone, has adverse effect on the weldability of the steel plate and the toughness performance of the welding joint, but also can greatly improve the yield ratio, the longitudinal and transverse strength anisotropy of the steel plate and the alloy cost (Ni is a noble alloy element) of the steel plate; therefore, the Ni content is controlled between 0.05% and 0.30%.
The addition of Mo can greatly improve the hardenability of the steel plate, promote the formation of bainite/martensite low-temperature transformation structure, improve the tempering characteristic and tempering process window of the steel plate, and improve the matching of the toughness and the plasticity of the steel plate after tempering; however, mo is used as a strong carbide forming element, and when Mo is excessively added, not only the low-temperature impact toughness, the elongation and the weldability of the steel plate are seriously damaged, but also the yield ratio, the longitudinal and transverse strength anisotropy and the production cost of the steel plate are greatly improved; therefore, comprehensively considering the influence of the transformation strengthening effect of Mo on the low-temperature toughness, elongation, weldability, yield ratio and anisotropy of longitudinal and transverse strength of the base steel plate, the Mo content is controlled between 0.08% and 0.25%.
The purpose of adding a trace amount of Nb into the steel is to perform unrecrystallized controlled rolling, refine the grain size of the steel plate and improve the strength and toughness of the TMCP steel plate, and when the Nb addition amount is less than 0.008%, the strengthening and toughening capability of the TMCP steel plate is insufficient besides the controlled rolling effect which cannot be effectively exerted; when the Nb addition exceeds 0.030%, not only the yield ratio, the longitudinal/transverse strength anisotropy of the steel sheet, but also the alloy cost of the steel sheet (Nb is also a noble alloy element) are kept high; and the formation of upper bainite (Bu) and secondary precipitation embrittlement of Nb (C, N) are induced under the condition of high heat input welding, so that the low-temperature toughness of a high heat input welding Heat Affected Zone (HAZ) is seriously damaged, the Nb content is controlled to be between 0.008 and 0.028 percent, the optimal rolling control effect is obtained, the protocol of TMCP steel plate toughness/strength-plasticity matching, low yield ratio and low longitudinal and transverse strength is realized, and the high heat input weldability of the steel plate is not damaged.
The purpose of adding trace Ti into the steel is to combine with N in the steel to generate TiN particles with high stability, and inhibit the growth of steel plate grains and welded HAZ region grains; the Ti content added in the steel is matched with the N content in the steel, and the matching principle is that TiN cannot be precipitated in liquid molten steel but must be precipitated in a solid phase; the precipitation temperature of TiN must therefore be ensured below 1400 ℃; when the addition amount of Ti in the steel is too small (less than 0.008%), the quantity of TiN particles formed is insufficient, and the formation of the TiN particles is insufficient to inhibit TMCP and the growth of austenite grains in the welding thermal cycle process so as to improve the low-temperature toughness and the weldability of the steel plate; when Ti content is excessive (> 0.016%), the TiN precipitation temperature exceeds 1400 ℃, and part of TiN particles are precipitated into large-size TiN particles in the molten steel solidification process, so that the large-size TiN particles can not inhibit the growth of crystal grains, but become starting points of crack initiation; therefore, the optimal control range of Ti content is 0.008-0.016%.
Als in the steel sheet can fix the free [ N ] in the steel]Reducing weld Heat Affected Zone (HAZ) free [ N]Improving the low-temperature impact toughness of the larger heat input welding HAZ; however, the addition of excessive Als to the steel not only forms a large amount of dispersed acicular Al in the steel 2 O 3 Inclusions, which impair the low-temperature impact toughness and weldability of the steel plate, are controlled to be between 0.040% and 0.070% according to analysis of a steel plate component system.
The control range of N corresponds to the control range of Ti, and Ti/N is preferably 1.5 to 3.5 for controlling the grain size of the steel sheet, improving the low-temperature toughness and weldability of the steel sheet. The N content is too low, the generated TiN particles are small in quantity and large in size, and the effect of controlling the steel plate grains cannot be achieved to improve the low-temperature toughness and the weldability of the steel plate, but the low-temperature toughness and the weldability of the steel plate are harmful; however, when the N content is too high, the free [ N ] in the steel increases, and particularly, the free [ N ] content in the Heat Affected Zone (HAZ) increases sharply under the condition of high heat input welding, thereby seriously impairing the low temperature toughness and bending cold workability of the HAZ and deteriorating the working and service characteristics of the steel. Therefore, the N content is controlled to be less than or equal to 0.0050%.
Ca treatment is carried out on steel, on one hand, molten steel can be further purified, on the other hand, the sulfide in the steel is denatured, so that the sulfide is changed into non-deformable stable and fine spherical sulfide, the hot shortness of S is restrained, the low-temperature toughness, the elongation and the Z-direction performance of the steel plate are improved, the anisotropy and the weldability of the longitudinal and transverse strength and the toughness of the steel plate are improved, and in addition, ca treatment is adopted, and the pouring of high-acid aluminum-soluble molten steel is improved; the addition amount of Ca depends on the content of S in the steel, the addition amount of Ca is too low, and the treatment effect is not great; the addition of Ca is too high, the formed Ca (O, S) is too large in size and the brittleness is increased, so that the Ca can become a fracture crack starting point, the low-temperature toughness and the elongation of steel and the weldability of a steel plate are reduced, and meanwhile, the purity of the steel and the pollution to molten steel are also reduced. The Ca content is controlled to be ESSP= (wt% Ca) [1-1.24 (wt% O) ]/1.25 (wt% S), wherein ESSP is sulfide inclusion shape control index, and the value is preferably in the range of 0.80-4.00, so that the suitable range of Ca content is 0.0010% -0.0035%.
The invention relates to a manufacturing method of a high-toughness, low-yield ratio and low-longitudinal and transverse strength anisotropic 600 MPa-level steel plate, which comprises the following steps:
1) Smelting and casting
Smelting according to the components, casting by continuous casting, and controlling the pouring superheat degree of a tundish to be 8-to-10
Controlling the pulling speed at 30 ℃ to be 0.6-1.0 m/min, and controlling the fluctuation of the liquid level of the crystallizer to be less than or equal to 5mm;
2) Slab heating
The heating temperature of the plate blank is controlled to be 1050-1150 ℃;
3) Rolling, wherein the rolling expansion ratio is more than or equal to 1.3
The first stage rolling is common rolling, and continuous rolling is carried out by adopting the maximum rolling capacity of a rolling mill;
in the second stage, rolling is controlled by adopting unrecrystallized rolling, the initial rolling temperature is controlled to 760-840 ℃, the rolling pass rolling reduction is more than or equal to 7%, the accumulated rolling reduction is more than or equal to 45%, and the final rolling temperature is 750-800 ℃;
4) Cooling
After rolling, the steel plate is immediately conveyed to an accelerated cooling device for accelerated cooling, the cooling temperature of the steel plate is 730-760 ℃, the cooling speed is more than or equal to 5 ℃/s, and the cooling stopping temperature is 250-500 ℃.
In the manufacturing process of the steel plate, the invention comprises the following steps:
according to the content ranges of C, mn, nb and Ti in the steel plate components, the heating temperature of the plate blank is controlled between 1050 ℃ and 1150 ℃, so that the austenite grains of the plate blank are not abnormally grown while the Nb in the steel is ensured to be completely dissolved into the austenite in the heating process of the plate blank, and the rolling expansion ratio is more than or equal to 1.3 in order to ensure that the steel plate has low longitudinal and transverse strength anisotropy.
The rolling adopts two-stage rolling, the first stage is common rolling, continuous rolling is carried out by adopting the maximum rolling capacity of a rolling mill, and the deformed billet is ensured to be recrystallized and austenite grains are refined while the productivity of a rolling line is improved to the greatest extent. In the second stage, non-recrystallization controlled rolling is adopted, according to the content range of Nb element in the steel, the rolling start temperature is controlled to 760-840 ℃, the rolling pass rolling reduction is more than or equal to 7%, the accumulated rolling reduction is more than or equal to 45%, and the finishing temperature is 750-800 ℃ in order to ensure the non-recrystallization controlled rolling effect.
After the controlled rolling is finished, the steel plate is immediately conveyed to an accelerated cooling device for accelerated cooling; the steel plate is cooled at 730-760 ℃ at a cooling speed of more than or equal to 5 ℃/s and at a cooling stopping temperature of 250-500 ℃, when the thickness of the steel plate is more than or equal to 50mm, the steel plate is slowly cooled, the slow cooling process is to keep the temperature at not lower than 250 ℃ for more than 24 hours, and then the steel plate is naturally cooled to room temperature.
The invention has the beneficial effects that:
the steel plate component design adopts a component system of low C-medium Mn-low (Cu+Ni+Mo) alloying-Nb microalloying-ultrafine Ti treatment by reducing the content of noble alloy elements, matching and combining main alloy elements, microalloying elements and inclusion elements, and is combined with a TMCP manufacturing process to produce the TMCP steel plate with excellent comprehensive performance at low cost, and the steel plate not only has high strength, high toughness, low yield ratio and low longitudinal and transverse strength anisotropy, but also has the yield strength of more than or equal to 460MPa, the tensile strength of more than or equal to 570MPa and the Charpy impact energy KV of minus 60 DEG C 2 The steel plate has the advantages of (single value) more than or equal to 120J, low yield ratio (less than or equal to 0.85), low longitudinal and transverse strength anisotropy (namely, the longitudinal/transverse strength difference is less than or equal to 30 MPa) and excellent weldability (no need of preheating before welding, welding heat input is more than or equal to 50 kJ/cm), and the steel plate has excellent weldability and can be welded with large heat input, so that the manufacturing period of the steel plate is greatly shortened, great value is created for construction enterprises, and the manufacturing and using processes of the steel plate are environment-friendly.
The high performance and high added value of the steel plate are concentrated on perfect matching of high strength, high toughness, low yield ratio, low longitudinal and transverse strength anisotropy and excellent weldability of the steel plate, and the problems of: (1) low C, low carbon equivalent CEV and high strength, low yield ratio; (2) high strength, high toughness and low yield ratio, low anisotropy of longitudinal/transverse strength; (3) the high strength and excellent weldability are conflicting with each other and difficult to reconcile in component design and TMCP process design, so that the safety, stability and durability of the large heavy steel structure are greatly improved. The good weldability (especially large heat input welding) saves the manufacturing cost of the user enterprise steel structure, shortens the manufacturing time of the user steel structure, and creates great value for the user.
Drawings
FIG. 1 is a gold phase diagram of the microstructure (1/4 thickness) of the steel of example 3 of the present invention.
Detailed Description
The invention is further described below with reference to examples and figures.
The compositions of the steel according to the present invention are shown in table 1, table 2 and table 3, which show the manufacturing process parameters of the steel according to the present invention, and table 4, which shows the performance parameters of the steel sheet according to the present invention.
Referring to FIG. 1, there is shown a gold phase diagram of the microstructure (1/4 thickness) of the steel of example 3 of the present invention. As can be seen from FIG. 1, the microstructure of the finished steel plate is uniform and fine ultra-low carbon bainite and a very small amount of massive ferrite, and the average grain size of the microstructure is below 20 mu m; low yield ratio (continuous yield phenomenon is generated in tensile test, rp0.2 is low), low longitudinal and transverse strength anisotropy (single ultra-low carbon bainite structure, and longitudinal and transverse strength anisotropy is greatly eliminated) are obtained.
With the rigid constraint of national economic development and carbon neutralization environmental indexes in China, the requirements of green conservation-oriented and harmonious society are established, infrastructure construction (such as large-scale public bridge girder engineering, municipal engineering, tunnel engineering and the like), ship manufacturing, ocean development, high-power ocean wind power engineering and major equipment manufacturing are put into daily life, and at present, the key materials of the infrastructure construction, ocean engineering development and major equipment manufacturing industry, namely the 600 MPa-grade steel plate with high toughness, low yield ratio and low longitudinal and transverse strength have wide market prospect.

Claims (4)

1. The high-toughness, low-yield ratio and low-longitudinal-transverse strength anisotropic 600 MPa-grade steel plate comprises the following components in percentage by weight:
C：0.060～0.10％
Si：≤0.20％
Mn：1.10～1.60％
P：≤0.013％
S：≤0.003％
Cu：0.05～0.25％
Ni：0.05～0.30％
Mo：0.08～0.25％
Nb：0.008～0.028％
Ti：0.008～0.016％
Als：0.040～0.070％
N：≤0.0050％
ca:0.0010 to 0.0035 percent; the balance of Fe and other unavoidable impurities; and the contents of the elements must satisfy the following relationships at the same time:
Als/N≥12；
[ξ×(900-T start rolling )×(％Nb)]/(H×ζ)≤2.25，
Wherein,,
h is the thickness of the finished steel plate, and the unit is mm;
xi is the cumulative rolling reduction of unrecrystallized controlled rolling in units;
T start rolling The initial rolling temperature of unrecrystallized controlled rolling is the unit DEG C;
ζ is the expansion ratio, namely the width of the finished steel plate/the width of the plate blank;
6≤[H×(T cooling -T Stop cooling )×CEV×ξ]/[(V c )×(T Stop cooling )]≤665，
Wherein,,
h is the thickness of the finished steel plate, and the unit is mm;
T cooling The cooling temperature is set to be the unit of the initial cooling temperature for accelerated cooling;
T stop cooling Stopping cooling temperature for accelerated cooling at unit ℃;
CEV is steel sheet carbon equivalent, cev=c+mn/6+ (cu+ni)/15+ (cr+mo+v)/5 in units;
xi is the cumulative rolling reduction of unrecrystallized controlled rolling in units;
V c the unit is that the cooling speed is accelerated for the steel plate;
ca treatment, and Ca/S ratio is controlled to be 1.0-3.0 and Ca X S 0.28 ≤1.5×10 －3 ；
The microstructure of the steel plate is uniform and fine ultra-low carbon bainite and a very small amount of massive ferrite, and the average grain size is below 20 mu m;
the yield strength of the steel plate is more than or equal to 460MPa, the tensile strength is more than or equal to 570MPa, and the impact energy KV at minus 60℃ is realized 2 The single value is more than or equal to 120J, the low yield ratio is less than or equal to 0.85, the low longitudinal and transverse strength anisotropy, namely the longitudinal/transverse strength difference is less than or equal to 30MPa, and the welding heat input is more than or equal to 50kJ/cm.
2. The method for manufacturing a high-toughness, low-yield-ratio and low-aspect-strength anisotropic 600 MPa-grade steel sheet according to claim 1, comprising the steps of:
1) Smelting and casting
The component smelting according to claim 1, wherein the casting is continuous casting, the pouring superheat degree of a tundish is controlled to be 8-30 ℃, the pulling speed is controlled to be 0.6-1.0 m/min, and the fluctuation of the liquid level of the crystallizer is controlled to be less than or equal to 5mm;
2) Slab heating
The heating temperature of the plate blank is controlled to be 1050-1150 ℃;
3) Rolling, wherein the rolling expansion ratio is more than or equal to 1.3
The first stage rolling is common rolling, and continuous rolling is carried out by adopting the maximum rolling capacity of a rolling mill;
in the second stage, rolling is controlled by adopting unrecrystallized rolling, the initial rolling temperature is controlled to 760-840 ℃, the rolling pass rolling reduction is more than or equal to 7%, the accumulated rolling reduction is more than or equal to 45%, and the final rolling temperature is 750-800 ℃;
4) Cooling
After rolling, the steel plate is immediately conveyed to an accelerated cooling device for accelerated cooling, the cooling temperature of the steel plate is 730-760 ℃, the cooling speed is more than or equal to 5 ℃/s, and the cooling stopping temperature is 250-500 ℃.
3. The method for manufacturing a 600MPa grade steel plate with high toughness, low yield ratio and low longitudinal and transverse strength anisotropy according to claim 2, wherein in the step 4), when the thickness of the steel plate is not less than 50mm, the steel plate is slowly cooled, the slow cooling temperature is not lower than 250 ℃, the temperature is kept for more than 24 hours, and then the steel plate is naturally cooled to room temperature.
4. The method for producing a high-toughness, low-yield-ratio and low-aspect-strength anisotropic 600 MPa-grade steel sheet according to claim 2 or 3, wherein the steel sheet has a microstructure of uniform and fine ultra-low-carbon bainite+a very small amount of bulk ferrite and an average grain size of 20 μm or less; the yield strength of the steel plate is more than or equal to 460MPa, the tensile strength is more than or equal to 570MPa, and the impact energy KV at minus 60℃ is realized 2 The single value is more than or equal to 120J, the low yield ratio is less than or equal to 0.85, the low longitudinal and transverse strength anisotropy, namely the longitudinal/transverse strength difference is less than or equal to 30MPa, and the welding heat input is more than or equal to 50kJ/cm.
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